1. Field of the Invention
The present invention relates to a liquid crystal displaying apparatus, and more particularly, to an active matrix type liquid crystal displaying apparatus.
2. Discussion of the Background
The array substrate of the conventional active matrix type liquid crystal displaying apparatus has a plurality of scanning wirings (or lines) in a row direction, and a plurality of signal wirings (or lines) in a column direction formed on an insulating substrate, thin film transistors (hereinafter,referred to as TFT(s)) formed in the intersecting positions of the scanning wirings and the signal wirings, and each pixel including a of pixel electrode connected with the TFT, an alignment film being formed on the pixel. Another substrate (hereinafter referred to as counter substrate) for interposing the liquid crystal has a common electrode on the insulating substrate, and an alignment film being formed on the common electrode. The array substrate and the counter substrate are confronted with each other in the face where the aforementioned pixel electrode and the common electrode are formed to interpose the liquid crystal composition in the gap between the array substrate and the counter substrate. As the alignment film is subjected to alignment process in a direction deviated by 90 degrees on the array substrate side and the counter substrate side, a TN liquid crystal is used with the liquid crystal molecule being twisted by 90 degrees in a thickness direction.
In such a TN type liquid crystal displaying apparatus, to prevent the leaking light from coming from the gap by removing the gap between the pixel electrode, the scanning wiring and the signal wiring in the periphery, and at the same time, to prevent disclination caused by the level difference of the pixel electrode end from the scanning wiring and the signal wiring and by the horizontal direction electric field between the pixel electrode and the scanning wiring or the signal wiring, there is disclosed such an art of forming a transparent insulating film with a film of 1 xcexcm or thicker and forming the pixel electrode on the scanning wiring and the signal wiring through the transparent insulating film. Thus, the disclination is prevented from being caused, and at the same time, the pixel electrodes can be superposed on the scanning wiring and the signal wiring so that the aperture ratio of one pixel can be made larger.
FIG. 33 is a plan view of one pixel of a liquid crystal displaying apparatus using an art to form, on the scanning wiring and the signal wiring through the transparent insulating film, the conventional pixel electrode described in, for example, Japanese Unexamined Patent Publication No. 258247/1997. FIG. 34 is a sectional illustrating view taken along a line of Axe2x80x94A of FIG. 33. Referring to FIGS. 33 and 34, reference numeral 1 is a transparent substrate, reference numeral 2 is a scanning wiring, reference numeral 3 is a storage capacitance line (hereinafter referred to as Cs line), reference numeral 4 is a gate insulating film, reference numeral 5 is a semiconductor layer, reference numeral 6 is a semiconductor layer with impurities being doped in it, reference numeral 6a is a source region for taking a signal wiring metal and an ohmic contact, reference numeral 6b is a region for taking a drain electrode metal and an ohmic contact, reference numeral 7 is a signal wiring, reference numeral 8 is a drain electrode, reference numeral 9 is a passivation film, reference numeral 10 is an organic transparent resin film, and reference numeral 11 is a contact hole (or contact via) for electrically connecting the drain electrode 8 with a pixel electrode. Reference numeral 12 is a pixel electrode where the position is shown with two-dot chain line. In the conventional liquid crystal displaying apparatus shown in FIGS. 33 and 34, these films normally form a resist pattern on the film by a process called a photolithography, and by removing the film of an unnecessary portion by etching of the resist pattern on the mask, the desired shape is obtained. To form the resist pattern by the photolithography process, an exposing machine is used. In the negative resist, the light is illuminated by shielding the light in the unnecessary portion of the film. In the positive type resist, the light is illuminated by shielding the light in the necessary portion of the film. Later, the resist of the unnecessary portion is removed by using a developing solution to form the desired resist pattern. In the exposing process in the photolithography, by using an exposing apparatus called a stepper, the displaying portion of the liquid crystal displaying apparatus is divided into some regions and the exposing is conducted for each region, whereby the whole displaying portion is exposed by several exposing operations. At this time, the exposing condition is changed in the step in the boundary of the adjacent exposing region by the results in the alignment accuracy of the exposing apparatus and the difference in uniformity of the exposing amount for each exposing region. Thus, the positional deviation between the patterns positioned in the different layers are changed in the step condition in the boundary of the adjacent exposing region. In the conventional liquid crystal displaying apparatus shown in FIG. 33, the drain electrode 8 also acts as an electrode for forming the storage capacitance. Accordingly, in the case of the conventional liquid crystal displaying apparatus shown in FIG. 33, when the pattern position of the scanning wiring 2 and the drain electrode 8 are deviated in the x axial direction of FIG. 33, the coupling capacitance Cgd formed by the superposition between the scanning wiring 2 and the drain electrode 8 in the TFT portion is changed and the relative position between the scanning wiring 2 and the drain electrode is changed, thereby changing the superposed area to change the value. The storage capacitance Cs is formed by the same photolithography process in the scanning wiring 2 and the Cs wiring 3. If the position of the drain electrode 8 is deviated in the x axial direction shown in FIG. 33, the superposing area of the Cs wiring 3 and the drain electrode 8 does not change, thus making the Cs value constant.
Influences of the change in the Cgd upon the picture quality will be described. FIG. 35 shows an equivalent circuit of one pixel of the liquid crystal displaying apparatus. For a simpler description, only the capacity composition of the present invention will be described. When an ON signal enters, the TFT 23 is turned on, whereby a prescribed electric charge is stored from the signal wiring 7 into the storage capacitance (Cs) 21 and the liquid crystal capacitance (Clc) 22. When the selection signal of the scanning wiring 2 is changed to off, the TFT is turned Off (high resistance condition), and the charge stored from the signal wiring is retained. The effective voltage to be decided by difference the between the electric potential to be decided by the electric charge and that of the Cs wiring (that is, common electrode of the counter substrate) is applied upon the liquid crystal, whereby the transmission ratio proportional to the effective voltage is obtained and the desired display is obtained. When the selecting signal of the scanning wiring 2 is changed, the electric potential of the drain electrode is changed by the coupling capacitance (Cgd) 24 between the scanning wiring 2 and the drain electrode 8. xcex94Vgd is as follows:
xcex94Vgd=(Cgdxc3x97xcex94Vg)/(Cgd+Cs+Clc)xe2x80x83xe2x80x83(1)
wherein the electric potential change of the drain electrode is xcex94Vgd.
The xcex94Vg is a change amount of the electric potential when the signal of the scanning wiring is changed from on to off. The center potential of the electric potential 25 (hereinafter referred to as Vcom) of the common electrode of the counter substrate and the center electric voltage of the voltage to be applied upon the liquid crystal are deviated by the electric change xcex94Vgd of the drain electrode. When the deviation causes the flickering (hereinafter referred to as flicker) of the picture, and a phenomenon (hereinafter referred to as image sticking) where the previous pattern remains even in the display switching when the same pattern is displayed continuously for long hours, thus deteriorating the display quality. The display quantity is prevented from being lowered normally by setting the center electric potential of the Vcom 25 to the xcex94Vgd.
In the case of the conventional liquid crystal displaying apparatus shown in FIG. 33, the Cgd 24 changes in the boundary of the exposing region, but the storage capacitance Cs21 does not change, whereby the xcex94Vgd changes for each exposing region. Since the center electric potential of Vcom 25 cannot be changed for each exposing region, a region, deviated from the center potential of the optimum Vcom 25, a region deviated from the center electric potential of the optimum Vcom 25, is caused without fail, and the flickering, image sticking and so on are caused, with a problem of deteriorating the display quality.
When the varying amount of the electric potential of the drain electrode by the xcex94Vgd is different for each exposing region, the effective voltage to be applied upon the liquid crystal is changed if the same signal electric potential is given, and the transmission ratio of the liquid crystal changes. When the xcex94Vgd is different for each exposing region, the transmission ratio becomes different among the regions, with a problem of visually recognizing them as a nonuniform display.
The present invention is conducted, considering the problems of the conventional active matrix type liquid crystal displaying apparatus. An object of the present invention is to provide a liquid crystal displaying apparatus of high display quality by preventing the display qualities such as flickering, image sticking, ununiformly displaying and so on from being reduced, which are caused by variation for each exposing region of the xcex94Vgd.
A liquid crystal displaying apparatus of the present invention comprises;
(axe2x88x921) a plurality of scanning wirings and a plurality of signal wirings to be arranged respectively in a row direction and a column direction,
(axe2x88x922) a TFT arranged in a matrix condition and controlled by the scanning wiring and the signal wiring,
(axe2x88x923) wherein the pixel electrode to be connected with the TFT is formed, the storage capacitance for retaining the electric charge is connected with the pixel electrode, another electrode opposite to the electrode for forming the storage capacitance and the drain electrode of the TFT are formed at the same time, and the scanning wiring and an electrode for forming the storage capacitance are formed by the same step,
(b) an array substrate with the pixel electrode superposed on the scanning wiring and the signal wiring through the transparent insulating film, and a display crystal displaying apparatus provided with an counter substrate of a common electrode arranged opposite to the pixel electrode, so as to compensate the change in the xcex94Vgd due to change in the Cgd to be caused for each exposing area by changing the Cs value for each exposing region.
A liquid crystal displaying apparatus of the present invention increases an area for forming the storage capacitance when the superposing area between the scanning wiring and the drain electrode, and an area for forming the storage capacitance also decreases when the superposing area between the scanning wiring and the drain electrode decreases.
A liquid crystal displaying apparatus of the present invention is adapted to change the Cs value so that (1) equation xcex94Vgd=(Cgdxc3x97xcex94Vg)/(Cgd+Cs+Clc) may become a constant value if the Cgd is changed.
In a liquid crystal displaying apparatus of the present invention, the transmission amount for transmitting the opening portion does not change even if the area of the storage capacitance is increased and decreased.
In a liquid crystal displaying apparatus of the present invention, at least one portion of the drain electrode is a transparent electrode.
In a liquid crystal displaying apparatus of the present invention, the width of the drain electrode is changed at steps or gradually to prevent the level difference cutting at the level difference of another electrode.
In a liquid crystal displaying apparatus of the present invention, the change in the aperture ratio, to be caused with increasing and decreasing in the area of the storage capacitance, may not be changed due to smaller light shielding area when the area of the storage capacitance increases, and larger light shielding area when the area of the storage capacitance become small.
In the conventional liquid crystal displaying apparatus, the xcex94Vgd changes for each exposing region, because the Cgd changes in the boundary of the exposing region, but the storage capacitance does not change. As the center electric potential of the Vcom cannot be changed for each superposing region, a region which is deviated from the center electric potential of the optimum Vcom is caused without fail to cause flickering, image sticking and so on, thus deteriorating the display quality.
When the varying amount of the electric potential of the drain electrode by the xcex94Vgd changes in each exposing region, the effective voltage to be applied upon the liquid crystal changes if the same signal electric potential is given, and the transmission ratio changes among the regions, which is visually recognized as a nonuniform display.
In a configuration having the storage capacitance for compensating the change of the Cgd in each exposing region of the present invention, the xcex94Vgd value becomes constant even if the exposing region is different, displaying quality reduction such as flickering, image sticking, ununiformly displaying and so on can be prevented, whereby a liquid crystal displaying apparatus is provided which is high in display quality, easy to manufacture and high in yield.
In the present invention, by having configurations for compensating the change in the aperture ratio in causing the Cs values for each exposing region, the quantity of the transmission light of one pixel of the liquid crystal displaying apparatus does not change if the Cs value changes. Thus, the xcex94Vgd is constant even if the exposing regions are different, quantity of the transmission light can be made unchanged, and the displaying quality reduction such as flickering, image sticking, nonuniform displaying and so on can be prevented. Thus, a liquid crystal displaying apparatus of better displaying quality which is free from nonuniform displaying by changing among the exposing regions in the quantity of the transmitting light can be obtained, wherein the manufacturing is easier and the yield is higher.
In the present invention, by using a transparent drain electrode, the quantity of the transmission light one pixel of the liquid crystal displaying apparatus does not change if the Cs value changes. Thus, the xcex94Vgd is constant and the quantity of the transmitting light does not change even if the exposing regions are different, the quantity of the transmission light can be made unchanged, and the displaying quantity reduction such as flickering, image sticking, nonuniform displaying and so on can be prevented. Thus, a liquid crystal displaying apparatus of better displaying quality where it is free from nonuniform displaying as the quantity of the transmitting light does not change among the exposing regions be obtained and the illuminance is high, wherein the manufacturing is easier and the yield is higher.
In the present invention, by improving the wiring shape for forming the drain electrode and the storage capacitance, and changing at steps or gradually, the drain electrode prevents the level difference cutting in the step difference to the wiring for forming the storage capacitance. Thus, the xcex94Vgd is constant even if the exposing regions are different, and the displaying quality reduction such as flickering, image sticking, nonuniform displaying and so on can be prevented. A liquid crystal displaying apparatus which is high in yield can be obtained.